Synthetic paper has been proposed and put into practical use in recent years to replace conventional natural pulp-made paper. The synthetic paper comprises a base layer made of a biaxially stretched polypropylene film containing 5 to 40 wt % of an inorganic finepowder, and paper-like layers disposed on the top and bottom surfaces thereof, made of a uniaxially stretched polypropylene film containing 8 to 65 wt % of an inorganic finepowder (e.g., as described in U.S. Pat. No. 4,318,950, Japanese Examined Patent Publication (KOUKOKU) Nos. 46-40794, 60-36173 and 62-35412).
Offset printing, (i.e., lithography), is generally used for printing on paper, polyethylene terephthalate film, polyamide film, coated paper and the like, since this technique can readily provide multi-color printing. The typical composition of a widely-used, drying-type offset printing ink is shown below.

TABLE 1Composition of Offset Printing Ink (weight part)Ink forInk forInk forsheetnon-paperrotary offsetprintingprintingpressPigment252525Resin253025Drying oil203410High-boiling-point,petroleum-base solvent25536Drier12—Others44 4Total100100100
The vehicle shown above refers to a liquid component, among the various components of which the printing ink is composed, which has the function of dispersing the pigment, thereby imparting the fluidity of the ink; ensuring the smooth flow of the ink from the ink pot to a printing surface while the ink is transferred among individual rollers, the printing plate and printing blanket; and for fixing the pigment onto the printing surface by solidification of the vehicle after printing. In response to the recent strong requirement that the offset printing ink have a shortened drying time to allow more rapid printing, there is an increasing trend toward the use of quick-drying inks which are compounded with a vehicle which is mainly comprised of a drying oil mixed with resin and mineral oil (high-boiling-point, petroleum-base solvent).
However, synthetic paper comprising a stretched polyolefinic film or such film compounded with an inorganic finepowder is printed with such quick-drying offset printing inks, vehicle of the quick-drying offset printing ink swells the polyolefin of the synthetic paper, thereby producing local surface waving or curling of the printed film. This is a significant practical obstacle in using such synthetic papers. Conventional offset printing inks for polyolefinic film are therefore specially formulated to be free from mineral oil. However, this approach sacrifices the quick-drying property.
In addition, such special offset printing inks for polyolefinic films require long drying times, and are used mainly in small specialty printing shops and manufacturers. There has therefore been a strong need for a new polyolefinic film on which the widely-used offset printing ink of the oxidation polymerization type (drying oil type) may be used.
In other words, in the typical printing shop, offset printing is practiced on pulp-based paper such as wood free paper and coated paper using generally-commercialized, quick-drying ink. Printing on polyolefinic film or synthetic paper in such printing shops requires the temporary replacement of such ink with the special offset printing ink which is suitable for such non-absorbent material. However, replacement of the conventional ink for pulp-based paper is time- and labor-consuming. As a consequence, conventional print shops have resisted printing on polyolefinic film or synthetic paper, which has been one factor preventing the general use of polyolefinic film or synthetic paper in offset printing.
One approach to solving this problem has been suggested in Japanese Unexamined Patent Publication (KOKAI) No. 8-333466, in which a polyolefinic resin mixed with an amorphous resin is used as a part of the multilayered film. However, this approach is still not completely successful in solving the problem of the swelling of the film (referred to as “solvent attack” hereinafter) due to the vehicle (in particular high-boiling-point, petroleum-based solvents such as mineral oil) in the offset printing ink. Another approach to solving the solvent attack has been suggested in Japanese Patent Unexamined Publication (KOKAI) No. 8-276540, in which a polyolefinic resin mixed with a petroleum resin is used to produce a multilayered film. However, this approach is not successful in solving the solvent attack in a practical level.
In view of the problems in the prior art, it is an object of the present invention to provide a stretched resin film having excellent printing properties, which is less prone to surface waving, even if the film is printed with widely-used, quick-drying offset printing ink.